Method for operating a vehicle powertrain

ABSTRACT

Methods and systems for controlling a vehicle powertrain that may be automatically stopped and started are presented. In one example, a method adjusts a position of a transmission clutch in response to battery current during engine cranking. The method may reduce clutch wear and improve vehicle launch from a stop.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 13/413,085, entitled “METHOD FOR OPERATING A VEHICLE POWERTRAIN,”filed on Mar. 6, 2012, the entire contents of which are herebyincorporated by reference for all purposes.

FIELD

The present description relates to methods and systems for controllingan engine that may be automatically stopped and started. The methods andsystems may be particularly useful during conditions to reduce clutchwear and improve vehicle launch after an automatic engine start.

BACKGROUND AND SUMMARY

An engine of a vehicle may be automatically stopped during selectedoperating conditions to conserve fuel. The engine may be subsequentlyautomatically restarted when operating conditions change, when a brakepedal is released for example. A transmission may also be coupled to theengine to deliver torque from the engine to vehicle wheels. In someexamples, the transmission may be an automatic transmission thatincludes few or no direct operator inputs to directly adjust theoperating state of the transmission. In other words, the operator maynot be able to manually control transmission clutches and gears.Instead, transmission clutches and gears may be adjusted by a controllerthat accepts inputs from the operator and ancillary sensors. If thetransmission is controlled in an undesirable manner during automaticengine stops and starts, the transmission may degrade at a rate that ishigher than is desired.

The inventors herein have recognized the above-mentioned disadvantagesand have developed a method for operating a vehicle powertrain,comprising: adjusting a position of a clutch of a transmission inresponse to a battery current during cranking of an engine whilestarting the engine.

By adjusting a position of a clutch in response to battery currentduring cranking of an engine while starting the engine, it may bepossible to reduce transmission degradation and improve vehicle launchduring automatic engine stops and starts. Specifically, battery currentmay be sensed during engine cranking and compared to a thresholdcurrent. If the battery current is greater than the threshold current,then the force being applied by the clutch may be reduced. On the otherhand, if less than a desired amount of engine torque is transferredthrough the transmission during engine cranking, then force applied bythe clutch may be increased to improve vehicle launch.

The present description may provide several advantages. For example, theapproach may reduce transmission degradation. Further, the approach mayimprove vehicle launch by reducing lag time between when the engine isstarted and when engine torque is delivered to vehicle wheels. Inaddition, the approach may improve battery life via reducing currentdraw from the battery during engine cranking.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example, referred to herein as the Detailed Description, when takenalone or with reference to the drawings, where:

FIG. 1 is a schematic diagram of an engine;

FIG. 2 is shows an example powertrain system layout;

FIGS. 3-4 are example schematic diagrams of transmission clutches;

FIG. 5 is a plot of an example engine operating sequence; and

FIG. 6 is a flowchart of an example powertrain control method.

DETAILED DESCRIPTION

The present description is related to controlling a vehicle powertrainduring engine stopping and starting. In one non-limiting example,transmission clutches may be adjusted in response to current suppliedfrom a battery during engine cranking. The vehicle powertrain mayinclude an engine as illustrated in FIG. 1. Further, the engine may bepart of a vehicle powertrain as illustrated in FIG. 2.

FIGS. 3 and 4 show example automatic transmission clutches. In oneexample, the clutches may be electrically actuated. However, in otherexamples the clutches may be hydraulically actuated. FIG. 5 shows anexample operating sequence when the method of FIG. 6 is executed via acontroller as shown in FIGS. 1 and 2.

Referring to FIG. 1, internal combustion engine 10, comprising aplurality of cylinders, one cylinder of which is shown in FIG. 1, iscontrolled by electronic engine controller 12. Engine 10 includescombustion chamber 30 and cylinder walls 32 with piston 36 positionedtherein and connected to crankshaft 40. Combustion chamber 30 is showncommunicating with intake manifold 44 and exhaust manifold 48 viarespective intake valve 52 and exhaust valve 54. Each intake and exhaustvalve may be operated by an intake cam 51 and an exhaust cam 53.Alternatively, one or more of the intake and exhaust valves may beoperated by an electromechanically controlled valve coil and armatureassembly. The position of intake cam 51 may be determined by intake camsensor 55. The position of exhaust cam 53 may be determined by exhaustcam sensor 57.

Fuel injector 66 is shown positioned to inject fuel directly intocylinder 30, which is known to those skilled in the art as directinjection. Alternatively, fuel may be injected to an intake port, whichis known to those skilled in the art as port injection. Fuel injector 66delivers liquid fuel in proportion to the pulse width of signal FPW fromcontroller 12. Fuel is delivered to fuel injector 66 by a fuel system(not shown) including a fuel tank, fuel pump, and fuel rail (not shown).Fuel injector 66 is supplied operating current from driver 68 whichresponds to controller 12. In addition, intake manifold 44 is showncommunicating with optional electronic air inlet throttle 62 whichadjusts a position of air inlet throttle plate 64 to control air flowfrom air intake 42 to intake manifold 44. In one example, a highpressure, dual stage, fuel system may be used to generate higher fuelpressures.

Ignition coil 88 provides an ignition spark to combustion chamber 30 viaspark plug 92 in response to a signal from controller 12. UniversalExhaust Gas Oxygen (UEGO) sensor 126 is shown coupled to exhaustmanifold 48 upstream of catalytic converter 70. Alternatively, atwo-state exhaust gas oxygen sensor may be substituted for UEGO sensor126.

Engine starter 96 may selectively engage flywheel 98 which is coupled tocrankshaft 40 to rotate crankshaft 40. Engine starter 96 may be engagedvia a signal from controller 12. In some examples, engine starter 96 maybe engaged without input from a driver dedicated engine stop/startcommand input (e.g., a key switch or pushbutton). Rather, engine starter96 may be engaged when a driver releases a brake pedal or depressesaccelerator pedal 130 (e.g., an input device that does not have a solepurpose of stopping and/or starting the engine). In this way, engine 10may be automatically started via engine starter 96 to conserve fuel.Starter 96 is supplied current via battery 80. Current flowing from orinto battery 80 is sensed via shunt resistor 85. Alternatively, element85 may be a coil for sensing battery current.

Converter 70 can include multiple catalyst bricks, in one example. Inanother example, multiple emission control devices, each with multiplebricks, can be used. Converter 70 can be a three-way type catalyst inone example.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, read-onlymemory 106, random access memory 108, keep alive memory 110, and aconventional data bus. Controller 12 is shown receiving various signalsfrom sensors coupled to engine 10, in addition to those signalspreviously discussed, including: engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling sleeve 114; a position sensor134 coupled to an accelerator pedal 130 for sensing force applied byfoot 132; a measurement of engine manifold pressure (MAP) from pressuresensor 122 coupled to intake manifold 44; an engine position sensor froma Hall effect sensor 118 sensing crankshaft 40 position; a measurementof air mass entering the engine from sensor 120; barometric pressurefrom sensor 124; and a measurement of air inlet throttle position fromsensor 58. In a preferred aspect of the present description, engineposition sensor 118 produces a predetermined number of equally spacedpulses every revolution of the crankshaft from which engine speed (RPM)can be determined.

In some examples, the engine may be coupled to an electric motor/batterysystem in a hybrid vehicle. The hybrid vehicle may have a parallelconfiguration, series configuration, or variation or combinationsthereof. Further, in some examples, other engine configurations may beemployed, for example a diesel engine.

During operation, each cylinder within engine 10 typically undergoes afour stroke cycle: the cycle includes the intake stroke, compressionstroke, expansion stroke, and exhaust stroke. During the intake stroke,generally, the exhaust valve 54 closes and intake valve 52 opens. Air isintroduced into combustion chamber 30 via intake manifold 44, and piston36 moves to the bottom of the cylinder so as to increase the volumewithin combustion chamber 30. The position at which piston 36 is nearthe bottom of the cylinder and at the end of its stroke (e.g. whencombustion chamber 30 is at its largest volume) is typically referred toby those of skill in the art as bottom dead center (BDC). During thecompression stroke, intake valve 52 and exhaust valve 54 are closed.Piston 36 moves toward the cylinder head so as to compress the airwithin combustion chamber 30. The point at which piston 36 is at the endof its stroke and closest to the cylinder head (e.g. when combustionchamber 30 is at its smallest volume) is typically referred to by thoseof skill in the art as top dead center (TDC). In a process hereinafterreferred to as injection, fuel is introduced into the combustionchamber. In a process hereinafter referred to as ignition, the injectedfuel is ignited by known ignition means such as spark plug 92, resultingin combustion. During the expansion stroke, the expanding gases pushpiston 36 back to BDC. Crankshaft 40 converts piston movement into arotational torque of the rotary shaft. Finally, during the exhauststroke, the exhaust valve 54 opens to release the combusted air-fuelmixture to exhaust manifold 48 and the piston returns to TDC. Note thatthe above is shown merely as an example, and that intake and exhaustvalve opening and/or closing timings may vary, such as to providepositive or negative valve overlap, late intake valve closing, orvarious other examples.

FIG. 2 is a block diagram of a vehicle powertrain 200. Powertrain 200may be powered by engine 10. Engine 10 may be started with an enginestarting system (as shown in FIG. 1). Further, engine 10 may generate oradjust torque via torque actuator 204, such as a fuel injector, airinlet throttle, etc.

An engine output torque may be transmitted to clutch 206 to drive anautomatic transmission 208 via transmission input shaft 236. Clutch 206may be comprised of one or more sets of clutch discs and one or morepressure plates as shown in FIGS. 3 and 4. Further, one or more gears230 coupled to first layshaft 278 or second layshaft 277 may beselectively engaged to propel a vehicle. In one example, the clutch 206may be referred to as a component of the transmission. The position ofclutch 206 may be adjusted to vary force applied by clutch 206 to coupleengine 10 to automatic transmission 208. Clutch 206 may be electricallyor hydraulically actuated.

Torque output from the automatic transmission 208 may in turn be relayedto wheels 216 to propel the vehicle via transmission output shaft 234.Specifically, automatic transmission 208 may transfer an input drivingtorque at the input shaft 236 responsive to a vehicle travelingcondition before transmitting an output driving torque to the wheels.

Further, a frictional force may be applied to wheels 216 by engagingwheel brakes 218. In one example, wheel brakes 218 may be engaged inresponse to the driver pressing his foot on a brake pedal (not shown).In the same way, a frictional force may be reduced to wheels 216 bydisengaging wheel brakes 218 in response to the driver releasing hisfoot from a brake pedal. Further, vehicle brakes may apply a frictionalforce to wheels 216 as part of an automated engine stopping procedure.

Gear clutches 230 may be selectively applied via gear actuator 233. Gearactuator 233 may be electrically or hydraulically operated. Clutch 206may be set to an open state when gear actuator 233 shifts betweendifferent gear ratios.

Transmission input speed may be monitored via transmission input shaftspeed sensor 240. Transmission output speed may be monitored viatransmission output shaft speed sensor 244. In some examples,accelerometer 250 may provide vehicle acceleration data to controller 12so that clutch 206 may be controlled (e.g., increase or decrease clutchapply pressure and adjust clutch engagement timing) via controller 12during engine starting and vehicle launch. In some examples, torquetransmitted through transmission 208 may be determined via a torquesensor 245.

A controller 12 may be configured to receive inputs from engine 10, asshown in more detail in FIG. 1, and accordingly control a torque outputof the engine and/or operation of the torque converter, transmission,clutches, and/or brakes. As one example, a torque output may becontrolled by adjusting a combination of spark timing, fuel pulse width,fuel pulse timing, and/or air charge, by controlling air inlet throttleopening and/or valve timing, valve lift and boost for turbo- orsuper-charged engines. In the case of a diesel engine, controller 12 maycontrol the engine torque output by controlling a combination of fuelpulse width, fuel pulse timing, and air charge. In all cases, enginecontrol may be performed on a cylinder-by-cylinder basis to control theengine torque output.

When idle-stop conditions are satisfied, controller 12 may initiateengine shutdown by shutting off fuel and spark to the engine. Further,to maintain an amount of torsion in the transmission, the controller maycontrol the force applied by clutch 206 so that the engine 10 is atleast partially engaged to vehicle wheels 216.

A wheel brake pressure may also be adjusted during the engine shutdownto limit vehicle movement during an engine shutdown. In one example, thewheel brake pressure may be adjusted to coordinate the application ofthe wheel brakes with the adjusting force applied by clutch 206 tocouple engine 10 with transmission 208. As such, by adjusting the wheelbrake pressure and the clutch application force, the amount of torsionretained in the transmission when the engine is shutdown may beadjusted.

When engine restart conditions are satisfied, and/or a vehicle operatorwants to launch the vehicle, controller 12 may reactivate the engine bycranking or rotating the engine and resuming cylinder combustion. Tolaunch the vehicle, application force of clutch 206 may be ramped up orincreased and the wheel brakes 218 may be released to increase torque tothe driving wheels 216.

Thus, the system of FIGS. 1 and 2 provides for a system for controllingan engine, comprising: an engine; a transmission including anelectrically actuated clutch; and a controller including executableinstructions stored in a non-transitory medium to automatically ceasecombustion in a cylinder of the engine and adjust the electricallyactuated clutch in response to a request to stop the engine, andinstructions to adjust a position, or pressure, or force, or torque, orcapacity, of the electrically actuated clutch in response to a batterycurrent during engine cranking while the engine is being started. Thesystem includes additional instructions to further adjust the position,or pressure, or force, or torque, or capacity, of the electricallyactuated clutch in response to a request to restart the engine beforethe engine stops in response to the request to stop the engine. Thesystem includes where the position of the electrically actuated clutchis opened to reduce force applied by the electrically actuated clutch inresponse to the request to restart the engine.

In some examples, the system also includes additional instructions toadjust the position of the electrically actuated clutch in response tothe battery current when the engine is at cranking speed and not toadjust the position of the electrically clutch in response to batterycurrent when the engine is at a speed that is different from crankingspeed by more than a threshold amount. The system includes where thecontroller includes further instructions to adjust the position of theclutch during engine cranking in response to an amount of torque outputby the transmission during engine cranking.

Referring now to FIG. 3 an example clutch 206 is shown. Clutch 206includes friction discs 318 which apply force to transfer torque toshaft 340 via spline 346. Damper spring 322 reduces oscillations throughthe transmission when force is applied to friction discs 318. Electricmotor 302 rotates screw 304 and causes roller 308 to move linearly inthe directions of arrows 350. Roller 308 acts on lever 312 to adjust theposition of pressure plate 316 as indicated by arrows 352. Return spring306 applies force opposing the force applied by electric motor 302 tolever 312 via roller 308. In this way, return spring 306 releasespressure plate 316 from applying force to friction discs 318 when roller308 is in the position shown. Lever 312 transfers force from electricmotor 302 to pressure plate 316 via engagement bearing 320.

Referring now to FIG. 4, an alternative example clutch 206 that includestwo sets of friction discs and two pressure plates is shown. Theclutches shown in FIG. 4 may be applied and released via an electricalactuator similar to that shown in FIG. 3.

In this example, clutch 206 includes a first set of friction discs 418and a second set of friction discs 419. Engine torque is transferredfrom the engine to first spline 446 when first pressure plate 402applies force to the first set of friction discs 418. In one example,first spline 446 transfers engine torque to a first layshaft. Similarly,engine torque is transferred from the engine to second spline 447 whensecond pressure plate 403 applies force to the second set of frictiondiscs 419. Damper springs 422 and 423 limit oscillations through thetransmission when engine torque is transferred to splines 446 and 447respectively.

Referring now to FIG. 5, a sequence showing example operation of themethod of FIG. 6 in the system of FIGS. 1 and 2 executed viainstructions stored in non-transitory memory of controller 12. Verticalmarkers at times T₀-T₇ indicate particular areas of interest during thesequence.

The first plot from the top of FIG. 5 is a plot of engine speed versustime. The X axis represents time and time increases from the left sideof the figure to right side of the figure. The Y axis represents enginespeed and engine speed increases in the direction of the Y axis arrow.

The second plot from the top of FIG. 5 represents transmission clutchapplication force. The X axis represents time and time increases fromthe left side of the figure to right side of the figure. The Y axisrepresents engine speed and engine speed increases in the direction ofthe Y axis arrow.

The third plot from the top of FIG. 5 represents battery current. The Xaxis represents time and time increase from the left side of the figureto the right side of the figure. The Y axis represents battery currentand battery current increases in the direction of the Y axis arrow.

The fourth plot from the top of FIG. 5 represents battery voltage. The Xaxis represents time and time increases from the left side of the figureto the right side of the figure. The Y axis represents battery voltageand battery voltage increases in the direction of the Y axis arrow.

The fifth plot from the top of FIG. 5 represents the clutch forcecommand. The X axis represents time and time increases from the leftside of the figure to the right side of the figure. The Y axisrepresents the clutch force command and the clutch force commandincrease clutch application force in the direction of the Y axis arrow.

At time T₀, engine speed is being reduced and clutch force is relativelyhigh as vehicle speed decreases. Battery current is low since fewaccessories are being powered. The battery voltage is high since thebattery is charged and since current demand is low. The clutch forcecommand is high to fully engaging the clutch.

Between time T₀ and time T₁, engine speed continues to be reduced andclutch force is reduced and vehicle speed approaches zero. The batteryvoltage remains high and the battery current remains low. The clutchforce command is reduced to reduce clutch application pressure.

At time T₁, an automatic engine stop request is issued to stop theengine. Since the engine is rotating, hydraulic pressure or electriccurrent may be applied to the clutch to adjust the clutch force. Theclutch force command is reduced to a predetermined level to prepositionthe clutch for a subsequent engine start. The clutch force decreases asthe clutch force command is decreased. The clutch force preposition isbased on a clutch position as determined from a previous engine start asdiscussed in more detail below. The battery voltage remains high and thebattery current remains low. Fuel and spark are deactivated and theengine stops rotating.

At time T₂, a request to automatically start the engine is issued. Theclutch command and the clutch force remain at the preposition level setat the engine stop. The engine starter is engaged and the engine beginsto rotate. The battery current increases as the starter engages, but inone example no adjustments are made to the clutch engagement in responseto the battery current when the starter is first engage. Rather,adjustments to clutch position are made based upon when the enginereaches cranking speed. In one example the engine reaches cranking speedwhen engine speed reaches a threshold engine speed. 250 RPM for example.Engine cranking speed may vary as compression work of the engineincreases and decreases during cranking. Thus, engine cranking speed mayvary between 200 and 300 RPM for example. However, clutch engagementforce is not adjusted in response to engine speeds outside the crankingspeed. For example, clutch engagement force is not adjusted based onbattery current when engine speed is between 0 and 150 RPM. Further,clutch engagement force is not adjusted based on battery current afterengine speed exceeds a threshold engine speed, 350 RPM for example.

At time T₃, engine speed is at cranking speed. Therefore battery currentis monitored and engine operating parameters are adjusted in response tobattery current after engine speed reaches a threshold engine speed. Inone example, engine cranking speed is the speed that the engine rotateswhen the starter is engaged for a predetermine amount of time at nominalbattery voltage when no combustion is present. The batter voltagedecreases somewhat but remains at a high level indicating a goodbattery. In cases where the battery voltage is reduced to less than athreshold voltage, clutch force is not adjusted in response to batterycurrent. In this example, the battery current exceeds predeterminedthreshold

At time T₄, engine speed exceeds engine cranking speed and thresholdspeed 502 which indicates the engine is started and then battery voltageand current are not used as a basis for adjusting clutch applicationpressure. The engine speed accelerates toward idle speed and the batterycurrent decreases. The battery voltage rises as the current demandfalls. The clutch force command remains constant as the engineaccelerates.

Between time T₄ and time T₅ the clutch force command is ramped up inresponse to an increasing engine torque demand from the operator. Theengine speed also increases as the operator increases the torque demandto accelerate the vehicle. The clutch force and clutch force command areshown being reduced several times. The clutch force is reduced whengears of the transmission are shifted. The engine speed increases anddecreases as the transmission is shifted. The engine speed decreaseswith vehicle speed as time T₅ is approached.

At time T₅, the vehicle and engine reaches conditions for automaticallystopping the engine (e.g., engine at idle speed, zero vehicle speed, andbrake pedal depressed) and the engine is automatically stopped. Beforethe engine is stopped, the clutch is prepositioned for engine startingby adjusting the clutch force command. In one example, the clutchapplication force is decremented from a previous setting duringautomatic engine stopping in response to the battery current greaterthan a threshold as shown between times T₃ and T₄. Thus, at time T₅clutch force is less than clutch force at time T₁ because batterycurrent exceeded the threshold 504 at time T₃. In some examples, theclutch force may be adjusted at the time the battery current ismonitored when battery current is greater than the threshold. Enginespeed is reduced to zero and battery current is at a low level.

At time T₆, a command to automatically restart the engine is issued andthe engine is cranked over. In one example, the engine is cranked via astarter with a pinion that engages a flywheel and rotates the engine. Inother examples, the engine may be rotated by an electric motor that iscoupled to the engine.

At time T₇, the engine reaches cranking speed and battery current ismonitored. Since battery current is less than threshold 504, batterycurrent is not adjusted. The battery voltage is at a relatively highlevel and the clutch command remains at the adjusted level which is lessthan the level applied during the last engine start at time T₂.

At time T₈, the engine speed exceeds the threshold engine speed 502 andso battery current is no longer monitored. The vehicle accelerates asdesired so the clutch application force is not increased duringpreposition in a subsequent engine restart. The engine speed increaseswith increasing torque demand and the transmission is shifted severaltimes between time T₈ and time T₉. Another automatic engine stop requestis issued at time T₉. The clutch position is adjusted to the samepreposition as the preposition at time T₅ since the battery currentduring the previous engine start was below threshold level 504.

Referring now to FIG. 6, a method for an example powertrain control isshown. The method of FIG. 6 may be executed by the system shown in FIGS.1 and 2 executing instructions stored in non-transitory memory. Themethod of FIG. 6 may provide the sequence shown in FIG. 5.

At 602, method 600 judges whether or not there is a request toautomatically stop the engine. If so, the answer is yes and method 600proceeds to 604. Otherwise, the answer is no and method 600 exits.

At 604, method 600 adjusts the transmission clutch to a force orpressure in response to the engine stop request. In other words, thetransmission clutch is adjusted to a preposition. The transmissionclutch force may be retrieved from memory. In one example, thetransmission clutch force is initially adjusted to a base amount and thebase amount is adjusted at 620 and 628 so that force applied to by theclutch is adapted over time. Method 600 proceeds to 606 after thetransmission clutch is prepositioned.

At 606, method 600 stops the engine. The engine may be stopped bydeactivating spark and fuel delivered to the engine. Method 600 proceedsto 608 after the engine is stopped.

At 608, method 600 judges whether or not there is an operator request tostop the engine. If so, the answer is yes and method 600 proceeds to610. Otherwise, the answer is no and method 600 proceeds to 612.

At 610, method 600 adjusts the clutch force the pressure plate appliesto clutch discs. In one example, the preposition force from 604 isreleased or at least partially released so that less force is applied bythe transmission clutch to couple the engine to the transmission gears.Transmission clutch force may be reduced when it is expected that theengine may be stopped for a longer period of time. Method 600 exitsafter the clutch preposition force is reduced.

At 612, method 600 judges whether or not there is an automatic enginestart request. If so, the answer is yes and method 600 proceeds to 614.Otherwise, the answer is no and method 600 returns to 608.

At 614, method 600 cranks the engine to start. In one example, theengine is cranked via a starter motor when a pinion of the starterengages a flywheel. In other examples, motors directly coupled to theengine rotate the engine. Method 600 proceeds to 616 after enginecranking begins.

At 616, method 600 monitors battery voltage, battery current, andambient temperature. Battery current may be monitored via measuringvoltage across a shunt resistor. In one example, the battery current maybe monitored when engine speed is greater than a first threshold speedand less than a second threshold speed. In one example, battery currentis monitored when engine speed is at cranking speed (e.g., 200-300 RPM).In other examples, battery current may be monitored when engine speedreaches a speed where fuel and spark are reactivated. For example,battery current may be monitored when engine speed reaches an idle speedof 800 RPM when the engine is rotated up to idle speed via a motor. Insuch examples, the battery current is monitored between engine speeds of750-850 RPM, for example. Battery voltage and ambient pressure are alsomonitored. If battery voltage is reduced to less than a threshold levelthe battery may be judged to be degraded and the transmission clutch isnot adjusted. Method 600 proceeds to 618 after battery currentmonitoring begins.

At 618, method 600 judges if battery current is greater than a thresholdcurrent level. If so, the answer is yes and method 600 proceeds to 620.If not, the answer is no and method 600 proceeds to 622. Further ifbattery voltage is less than a threshold, method 600 proceeds to 622whether or not battery current is greater than a threshold current.

At 620, method 600 commands a decrease in the transmission clutchapplication force. The transmission application force may be reduced viareducing force applied by a pressure plate to clutch discs. In otherexamples, hydraulic pressure supplied to actuate the clutch may bereduced. The command may adjust transmission clutch pressure during thepresent engine start. Additionally, the command lessens the forceapplied by the transmission clutch during engine cranking during asubsequent engine restart. If the engine temperature is low, the clutchapplication force may be increased at a reduced rate. For example, ifengine temperature is low and battery current exceeds a threshold,transmission clutch application force may be reduced at a rate that islow than if engine temperature is high. The reduced decrement totransmission clutch force may be used to account for additional enginefriction at lower engine temperatures that may increase battery current.Method 600 proceeds to 622 after the clutch preposition command isadjusted.

At 622, method 600 judges whether or not the engine is started or ifengine speed is exceeding a threshold engine speed. If so, the answer isyes and method 600 proceeds to 624. Otherwise, the answer is no andmethod 600 returns to 616.

At 624, method 600 begins transferring engine torque to the vehiclewheels via increasing transmission clutch application force. Thetransmission clutch application force may be increased via commanding anelectric motor or increasing pressure of hydraulic fluid that actuatesthe transmission clutch. Method 600 proceeds to 626 after increasingtransfer of engine torque to vehicle wheels.

At 626, method 600 judges whether or not engine torque has beentransferred to vehicle wheels at a desired rate. In one example, if theis a delay between when the transmission clutch force is increased andwhen additional torque is transferred to vehicle wheels, it may bedetermined that the transmission clutch force set during prepositioning(e.g., at time of an automatic engine stop request) is too low. Ifmethod 600 judges that engine torque is not supplied to vehicle wheelsin a desired way, the answer is no and method 600 proceeds to 628.Otherwise, the answer is yes and method 600 proceeds to exit.

At 628, method 600 increases the transmission clutch preposition forcerequest that is commanded in response to an automatic engine stoprequest, at 604 for example. The increase in clutch force may be storedin memory and retrieved during a subsequent engine start. In oneexample, the transmission clutch application force is increased by apredetermined amount, 0.5 N-m for example. Method 600 proceeds to exitafter the transmission clutch force is increased.

In this way, the transmission clutch prepositioning force applied inresponse to an automatic engine stop request may be increased ordecreased depending on battery current and torque transfer from theengine to the vehicle wheels. Thus, method 600 provides a method foroperating a vehicle powertrain, comprising: adjusting a position of aclutch of a transmission in response to a battery current duringcranking of an engine while starting the engine. The method includeswhere the clutch mechanically couples output of an engine to a layshaftof the transmission. The method also includes where the clutch is atleast partially engaged and transferring torque from the engine to thelayshaft.

In some examples, the method includes where the clutch is anelectrically actuated clutch. The method further comprises adjusting theposition of the clutch in response to battery voltage and ambienttemperature. The method includes where battery current is determined viaa shunt resistor. The method also includes where adjusting the positionof the clutch is in response to the battery current after the enginereaches cranking speed and not in response to battery current before theengine reaches cranking speed during engine cranking.

Method 600 also provides for operating a vehicle powertrain, comprising:adjusting a clutch of a transmission to a first position in response toa request to automatically stop an engine; stopping the engine;adjusting an control parameter of the transmission in response tobattery current during cranking of the engine during a first enginerestart; and adjusting the clutch to a second position in response tothe control parameter. The method includes where adjusting the clutchoccurs during a second engine restart. The method includes where theclutch is an electrically actuated clutch. The method also includeswhere the clutch remains at the first position during the engine stop.

In some examples, the method includes where the control parameter isadjusted in response to battery current after the engine reachescranking speed and not in response to battery current before the enginereaches cranking speed. The method also includes where the controlparameter is adjusted to reduce clutch application pressure in responseto battery current exceeding a threshold. The method also includes wherethe control parameter is adjusted to increase clutch applicationpressure in response to a transmission output torque less than a desiredtransmission output torque during cranking. The transferred enginetorque may be measured or inferred. The method includes where thecontrol parameter is a clutch application force, where the clutchapplication force is further adjusted in response to a temperature ofthe engine, and where clutch application force is increased at a reducedrate as the temperature of the engine decreases.

As will be appreciated by one of ordinary skill in the art, routinesdescribed in FIG. 6 may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various steps orfunctions illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the objects, features, andadvantages described herein, but is provided for ease of illustrationand description. Although not explicitly illustrated, one of ordinaryskill in the art will recognize that one or more of the illustratedsteps or functions may be repeatedly performed depending on theparticular strategy being used.

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas,gasoline, diesel, or alternative fuel configurations could use thepresent description to advantage.

The invention claimed is:
 1. A method for operating a vehiclepowertrain, comprising: adjusting a transmission clutch position inresponse to a battery current during engine cranking while starting anengine and in response to battery voltage and ambient temperature. 2.The method of claim 1, where a transmission clutch mechanically couplesan output of an engine to a layshaft of a transmission.
 3. The method ofclaim 2, where the transmission clutch is at least partially engaged andtransfers torque from the engine to the layshaft.
 4. The method of claim2, where the transmission clutch is an electrically actuated clutch. 5.The method of claim 1, where battery current is determined via a shuntresistor.
 6. The method of claim 1, where adjusting the transmissionclutch position is in response to the battery current after the enginereaches cranking speed and not in response to battery current before theengine reaches cranking speed during engine cranking.